Since the introduction of generic drugs to the pharmaceutical market a sometimes emotional debate exists whether they are well-investigated and of high quality. There is some uncertainty about whether evidence of bioequivalence is enough to guarantee efficacy and safety of generic drugs. Some physicians ask the question if competent authorities are able to ascertain that the pharmaceutical quality of generics is acceptable. Doctors and patients sometimes are ill at ease about the interchangeability of innovator and generic products. This article describes how the European Union legislation ensures that a generic drug is only approved if its risk-benefit relationship is favourable and that it is essentially similar to the innovator product. In this context pharmacokinetic parameters are accepted as surrogates for clinical results because bioequivalence means therapeutic equivalence as well. For most drugs, current bioequivalence testing generally enables clinicians to routinely substitute generic for innovator products. Published findings, however, suggest that particular drugs may not be ideally suited for generic substitution when a patient is already on that drug. These are the so called critical dose medicinal products (drugs with a narrow therapeutic range). When starting a new therapy with any generic drug, however, its similarity to the innovator drug in terms of efficacy, safety and quality is guaranteed.